Oxidation of sulfide deposits containing copper values

ABSTRACT

Recovery of copper values by in-situ mining achieved by injecting a gaseous mixture of oxygen and steam and producing a lixiviant optionally with addition of sulfur trioxide or sulfur dioxide; recovery of copper values at moderate and great depth can be achieved.

Unlted States Patent 1191 1111 3,881,774

Van Poolen et al. May 6, 1975 [54] OXIDATION OF SULFIDE DEPOSITS 2,695,163 11/1954 Pearce et al. 166/261 X CONTAINING COPPER VALUES 2,839,141 6/1958 Walter 166/261 3,278,233 lO/l966 Hurd et al..... 299/4 Inventors: Hendrik Karel Van Pwlen, 3,410,604 11/1968 White et a1... 299 4 Littleton, Col0.; Ray Vincent Huff, 3,532,165 10/1970 Raifsnider..... 166/270 Acton, Mass. 3,640,579 2/1972 Lewis r 299/4 3,708,206 l/l973 Hard at al.. 299/5 Asslgneel Kenneco" Copper Corporatwn, 3,775,097 11 1973 Cech 299 5 x New Y NY. 3,823,981 7/1974 Lewis 299 4 [22] Filed: Apr. 18, 1974 [21] pp No 461 902 Primary ExaminerStephen J. Novosad [52] US. Cl. 299/4 [57] ABSTRACT I z q S e at ch 33; 2 a 5 9 Recovery of copper values by m-sltu mmmg achieved by injecting a gaseous mixture of oxygen and steam and producing a lixiviant optionally with addition of sulfur trioxide or sulfur dioxide; recovery of copper [56] References cued values at moderate and great depth can be achieved.

UNITED STATES PATENTS 2,563,623 8/1951 Scott 299/5 X 10 Claims, No Drawings OXIDATION OF SULFIDE DEPOSITS CONTAINING COPPER VALUES This invention pertains to recovery of minerals such as copper from ore formations lying at great depths; more particularly, this invention pertains to in-situ mining of minerals such as copper. Still further, this invention pertains to the promotion or stimulation of a wellbore used for the in-situ mining by working the surrounding rock formation for recovery of copper values by the use of steam admixed with oxygen and injected in the wellbore.

BRIEF DESCRIPTION OF THE BACKGROUND OF THE INVENTION In the mining of copper, generally two techniques have been employed. The first is the open pit mining well known in the Western United States. The second method is the conventional underground mining by shafts and tunnels whereby the ore body is being worked underground and the ore rich in copper values is then sent above ground for processing.

It has become increasingly more expensive to mine copper by the second method which must be practiced whenever the ore body lies at great depths. Typically, ore bodies from which the overburden cannot be removed are worked by the underground mining techniques. However, the underground mining techniques become extremely expensive when it is remembered that the average copper value in the best ores reaches only about 3 percent based on weight of the ore. With the increasing scarcity of copper bodies which can be mined by the open pit mining technique, and with the increasing economic penalties associated with underground mining, it has become necessary to seek alternate sources or utilize new techniques for obtaining copper. In-situ mining is applicable not only to the cop per oxide mining but also native copper, and various sulfidic copper ore mining.

According to the in-situ mining, which has been increasingly looked upon as a viable economical alternative to the underground mining, it has become fairly evident that the recovered rate of the copper values, however, must be boosted before the in-situ mining technique can present itself as an economically feasible method.

BRIEF DESCRIPTION OF THE INVENTION It has now been discovered that, in the art of in-situ mining of copper, various means have to be employed in order to render the recovery process economically feasible. Although for the in-situ mining process, numerous theoretical suggestions or approaches have been proposed on how to maximize the recovery rate of copper, depending on the mineral content of the ore body, the most desirable or best methods because of the dearth of industrial experience are yet to be worked out. Nevertheless, it is fairly evident that in the recovery of copper values, in order for the process to be economical, all elements in the recovery process must be coacting in such a manner as these would perform at least more than one function. Inasmuch as the multifunctionally acting elements in the recovery process render these processes that much more feasible, a practical in-situ mining process will become more of a widespread industrial reality.

According to the present invention, it has now been discovered that if steam is injected in a wellbore in conjunction with oxygen, the oxidation of the chalcopyrite (CuFeS or other metallic sulfides such as pyrites (FeS covellite (CuS); bornite (Cu FeS chalcocite (CuS renders the metal values soluble in acid solutions. Enargite-Cu AsS tetrahedrite-Cu SbS and tennantite-Cu As S can also be treated by the method herein, provided the rock in which these copper values are found is leachable. Inasmuch as a product of oxidation in the wellbore is sulfur dioxide and/or sulfur trioxide, which upon steam condensation will form an acid solution, the oxygen-steam injection provides thus a noteworthy multiple functionality for the injected steam-oxygen gas.

As a result of the acid solution, a lixiviant formation in a proportion suitable for removal of metal values is achieved in-situ; while at the same time, the metal values become that much more accessible to the lixiviant. Still further, it has now been found that as the result of steam and oxygen injection, the mixture remains homogeneous which is of great advantage as multiphase reaction, mass transport, boundary layer etc. phenomena are eliminated and/or reduced.

Essentially, a gaseous system is being used throughout the wellbore and thus associated multiphase phenomena are avoided. Still further, the heat which is imparted to the system by using steam injection also enhances the oxygen-sulfide reaction. Moreover, additional quantities of sulfur trioxide in a gaseous form can be injected with the oxygen steam mixture thereby further enhancing the lixiviant properties of the end product of the in-situ reaction.

A wellbore worked by the in-situ technique disclosed herein may be cased and sealed by techniques well known in the art.

BRIEF DESCRIPTION OF PRIOR ART In US. Pat. No. 3,405,761, it has been disclosed that simultaneous or separate injection of steam and carbon dioxide in a well produces noteworthy results. Still further, in US. Pat. No. 3,532,165, the formation of carbon dioxide in-situ by the reaction of sodium carbonate and sulfuric acid has been disclosed. However, in this method, the cost justification for using an approach such as disclosed in this patent cannot be employed for in-situ mining of copper values for a number of reasons including the formation of undesirable salts. Still further, in US. Pat. No. 3,640,579 oxygen displacement of hydrostatic water in a flooded nuclear chimney has been disclosed. However, the system disclosed in this patent requires a fairly well rubblized chimney having dimensions and flow rates not encountered in-situ mining where permeability is of a magnitude significantly smaller. Hence, the proposed method is inapplicable for wellbore stimulation, although the removal of leachable compounds could increase permeability. In US. Pat. No. 3,708,206, in addition to using oxygen as a gas, an oxygen foamed leach solution is pressurized into a formation to oxidize uranium from one valance state to another and thus render the thus oxidized uranium soluble. However, a two phase introduction of oxygen is unattractive for a number of reasons such as dispersion, phase separation, depletion of oxygen, etc.

DETAILED DESCRIPTION OF THE INVENTION In attempting to bring a suitable dispersion of oxygen into a wellbore when employing an aqueous solution, numerous adverse conditions apply; and these have heretofore mitigated the use of oxygen in an aqueous fluid. For example, elaborate methods and/or equipment may be necessary to obtain a suitable dispersion of oxygen as a gas dispersed in an aqueous fluid, whereby the injected oxygen stays in a proper quantity in the aqueous fluid. Still further, the dispersion of oxygen must be sufficiently well distributed and the bubbles of oxygen must be sufficiently small so that these may enter the pores or fracture apertures in the rock. Still further, the quantity of oxygen should be evenly distributed throughout an entire ore column which is being worked by the in-situ method. These considerations have made it almost intolerable from the standpoint of complexity, economics and utilization of oxygen in a liquid phase or dispersed gas phase. Inasmuch as the recovery values from a two-phase flow and improperly distributed oxygen in a wellbore reservoir is a variable function, it is necessary to improve the uniformity so that recovery control may be monitored with greater assurance.

According to the present invention, it has been found that if steam is used instead of water or other dilute aqueous solutions, only one-phase injection is necessary. The injected fluid being in one-phase readily passes and penetrates the ore body being worked such that the recovery of the proper values in the ore is enhanced by the conjoint penetration of steam and oxygen in the pores, and the subsequent condensation of steam in pores causes the above-mentioned lixiviant formation.

In accordance with the present invention, the chemical representation of the various competing reactions in a copper containing ore body is illustrated as follows:

In addition, as mentioned before, the injection of sulfur trioxide produces the following competing reaction further enhancing the present method.

As lithostatic pressure is generally higher than hydrostatic pressure for a given depth, the pressure of a steam plus oxygen must be only slightly higher than hydrostatic pressure (about 435 psi for each 1,000 feet) before the ore can be permeated by the gaseous mixture. The rate at which the mixture can be injected is a function of differential pressure, permeability, etc. and can be determined using Darcys law. In order to avoid shortcircuiting, the rock parting pressure should not be exceeded.

It is well established that oxygen is miscible with steam in all proportions. However, the desired ratio of water to oxygen on a weight basis is in the range of 30:1 to 95:1. Lower ratios may also be used, e.g. 25:], but without significant advantage. The exact amounts depend on several parameters including the ore grade, pyrite to chalcopyrite ratio, mineral composition of rock, etc. The prime consideration, of course, is to strive for maximum economical copper loadings. It is desirable to inject along with the steam and oxygen some quantity of sulfur trioxide or sulfur dioxide which would effectively provide more acid.

There are certain limitations imposed on the process because of physical limitations of steam generators, tubular goods, and well completion techniques. However, it is not beyond the scope of this invention to leach at depths in excess of 5,000 ft. This process relies upon the vapor pressure of steam to overcome the hydrostatic deposit pressure. If the water table is depressed, obviously the hydrostatic pressure in the deposit is reduced and a lower steam temperature will be required to develop the injection pressure.

The reaction rates of sulfides with oxygen are enhanced with increased temperature, although even at lower temperatures, the reaction rates are sufficiently adequate. It is conceivable that reaction rates can be catalyzed or inhibited as the process may require, although neither is thought to be required.

For purposes of an illustrative embodiment, the bottom of an ore deposit terminates at 4,000 feet from the surface. Further, the water table is at (or within a few feet of) the surface. The hydrostatic pressure that would exist at 4,000 feet, if the water contains no dissolved solids, is 0.433 X 4000 or 1732 psi. Therefore, to develop a pressure of 1732 psi, water must be converted to steam at a temperature of about 615F. However, if the gas (steam plus oxygen) is composed of 5 percent oxygen on a volume basis, then the partial pressure of steam required is about 1645 psi and corresponding steam temperature of about 608F. As the fraction of steam is decreased, the partial pressure of steam is decreased; and therefore, the temperature of the injected fluid may be decreased but a constant bottom hole pressure can yet be maintained.

In accordance with the above invention, the following data is illustrative of an oxygen steam injection process whereby the lixiviant production in-situ is achieved. The embodiment, which is illustrated herein is merely for the purpose of teaching the operative mode of the invention; and other embodiments cumulative thereof can be easily envisioned therefrom.

At least one well is drilled into an ore deposit and completed. Generally, a five hole pattern is used, that is, an injection hole in a center and four producing holes one in each corner of a square. With an adequate source of water and oxygen, the water is fed into one or more steam generators at a rate consistent with the injection requirements of the project such as based on permeability (cf. Darcys law) and lixiviant loading. Water is converted to steam and heated to the temperature necessary to develop the required injection pressures. All or only a fraction of the water (depending on water quality) is converted to steam. Depending upon the well completion, the oxygen and steam may be commingled at the surface and simultaneously injected throughout a single tubing string to the mining interval; or the steam and oxygen may be introduced to the bottom of the well through separate tubing strings and admixed for injection into the deposit. In addition, 50:; or other acid forming gases may be injected with the steam and/or oxygen to provide a suitable lixiviant. To avoid corrosion or other deleterious effects, other additives may be incorporated into the injected fluid. The

tubular goods may be insulated to reduce heat loss from the injection fluid to the rock prior to the injection fluid arriving at the ore zone of interest.

As the injected fluid enters the deposit, oxygen reacts with the mineral values to form readily soluble compounds. As illustrated by the reactions above, the steam condenses and forms acid with the acid-forming gases or deposits. The liquid acid then dissolves the metal values.

This process is operable through a single well in a huff-puff mode, that is, by injecting a batch of oxygen and steam and then producing the pregnant liquor from the same well. Or the process is operable using several wells, at least one injection, and one productionwell. Further, the process is operable with several wells but alternating the injection and production wells, e.g., the above discussed five hole pattern of numerous wells in a field.

In general, the amount of sulfur trioxide which may be used is back calculated from the amount of sulfuric acid needed (based on steam and oxygen requirements by weight). This amount is based on the rate of acid introduced, plus the acid generated by oxygen with the sulfide minerals, less the acid consumed by the acid reacting with gangue minerals.

As described above, the process is operable at a medium or great mineral site depths and depends on its proper practice in a single hole event as limited by the rock parting pressure (to avoid short circuiting in rock) as an upper pressure limit. The lower and practical pressure is defined by the hydrostatic head of water at the given mineral deposit. Between these values, the advantageous pressures are established by Darcys law, i.e. permeability considerations.

In a two hole or multi hole situation, the pressure is dependent on the steam pressure used in the injection well (above the hydrostatic pressure) and the rate of withdrawal of lixiviant from the production well. At steady-state conditions, the pressure will be the average of the two pressures in these two wells (or average of any multiple of injection and production wells).

The necessary steam temperature and pressures, as a function of steam partial pressure and oxygen partial pressure within a ratio of steam and oxygen have been explained above. Inasmuch as the hydrostatic pressure defines the steam temperature without oxygen, and inasmuch as the total pressure of steam and oxygen (and 50 for any operating condition is known, calculation of steam pressure can be easily accomplished by reference to Steam Tables and as illustrated above. Any pressures employed above the hydrostatic pressure thus define the temperature for steam; and any selected amount of oxygen defines the partial pressure of steam and thus the steam temperature necessary. As it can well be appreciated, the cooling effects (previously indicated re insulated pipe) are compensated for by an appropriate determination of pressure drop (in a closed pipe string). Generally a suitable over pressure is sufficient, e.g. 5 percent to compensate for the various losses in pressure due to cooling.

It will also be understood that the exact phenomena at the bottom of the wellbore are deducted by experiments, by deductions and interpretations of observed data, etc. Because of the great depth, the exact mechanism may be different. However, the above explanations are for the purpose of illustrating the invention; and a correct theory is not necessarily espoused by the explanation as the true state of the facts may be different. The observed data are to be considered at all times in this light.

The lixiviant loading and the competing chemical reaction considerations have been further explained below and illustrate the background considerations with respect to in-situ mining metal values.

Rock associated with porphyry-copper deposits generally will contain acid consuming minerals. These minerals may represent 5 to 30 percent of the rock.

A list of acid consuming minerals includes minerals such as calcite and biotite. If biotite is completely reacted with H then a theoretical maxium of 1.5 gram H SO will be consumed per gram of biotite. However, experimental evidence suggests that crushed rock containing biotite (which represents most drastic acid consumption) requires 0.6 gram H 80 to react with each gram of biotite.

On the other hand, sulfide minerals when oxidized produce acid. Pyrite (Py) and chaleopyrite (Cpy) are the principal minerals. Acid production for these minerals is listed below.

Minerals Ratio Py/Cpy gm H 50 generated/per gm Copper Cpy 0 0.75 0

Cpy:Py 1 2.4 1.6

Cpy:Py 2 7.0 3.2

Basis:

Required H 50 X .20 X .6 100 X .006 X 2.4

= 12 1.44 1056 grams or 0.106 grams acid per gram of rock.

Reactions:

H O SO I-I SO or because oxygen is present H O S0 H2 O H SO In view of the above, for each gram of H 80, one needs 0.82 grams of S0 One cubic foot of rock weighs about 165 pounds or about 75,000 grams. Therefore, each cubic foot of rock will require 7950 grams of H SO A suitable lixiviant contains oxygen, water, and sulfuric acid. The condensed liquid phase (about 20% H 80 contains about 200 grams of the H 80 per liter of liquid, and the combined phases have a ratio of about 8 grams of 0 per liter of liquid.

Consequently, the proportion of fluids entering the well head of the injection well during unit time is 937 grams H 0, 163 grams S0 and 8 grams of 0 From the International Critical Tables, the vapor pressure for 20 percent H 80 is given by the following equation (this equation should be accurate up to C):

2268 log p 8.922

p pressure in mm of Hg T temperature in K where maintain the fluid in the gaseous state for any given pressure.

As the wellbore may be cased for any given depth, working of the ore body may also be for a depth confined to the thickness of the ore body or fractional thickness of the ore body; hence, the employed pressure and temperature conditions are generally based on the stratum being worked and its lower depth. An illustrative ore body which is capable of being worked by the method described herein is found near Safford, Arizona.

At depths of less than 1500 feet, the geostatic pressure will determine the rock parting pressure because of vertical and horizontal fracturing. Generally, the rock parting pressure will then be less than 0.5 psi/feet of gradient which can then be assumed as an approximate upper useful limit appropriately reduced by the usefully employed pressures above hydrostatic pressure.

As it is well known in the art how copper can be recovered from the lixiviant solutions, the copper recovery process is not set forth herein. Similarly, the wellknown wellbore preparation, the lixiviant removal from the wellbore and its reconsitution, if needed for reuse, are not discussed.

Darcys law is defined in Petroleum Production Handbook, Thomas C. Frick, Ed., Vol. II, McGraw-Hill Book Company, New York, N.Y.

The permeability values for copper ore bearing deposits, as it is well known, are however, significantly smaller than the permeability values associated with oil bearing rocks. Consequently, the problems associated with penetration without rock parting and the introduction of a gaseous medium bearing all the reactants (substantially without phase separation) provides the manifold advantages herein.

Rock parting pressure is defined for sedimentary rocks below 1500 feet (because of substantially vertical fracturing) as ranging between 0.7 psi/ft. gradient to 0.5 psi/ft. of gradient.

What is claimed is:

I. In a process for recovering metal values by in-situ mining of same, the steps comprising: drilling at least one wellbore in an ore body having metallic sulfide values; injecting mixture of steam and oxygen in said wellbore in proportions of at least 25:1 ratio by weight of steam to oxygen as a gas whereby the gas is at least at the hydrostatic but below the rock parting pressure required for gas injection and the temperature, depending on the composition, is consistent with the injection pressure, said in-situ mining being carried out at a depth preferably beneath the water table; maintaining a mixture of steam and oxygen in an intimate contact in said ore body adjacent said wellbore; oxidizing said metal sulfides in said ore body whereby sulfur, sulfur dioxide and/or trioxide are fonned and subsequently converted to sulfuric acid with said steam, said sulfur dioxide and/or trioxide thereby forming an acid lixiviant solution in said wellbore; maintaining said lixiviant solution in intimate contact with said metal values in said ore formation, and recovering said metal values together with said lixiviant and condensed steam.

2. The process according to claim 1 and wherein additional amounts of sulfur trioxide are injected in said wellbore in admixture with steam and oxygen.

3. The process as defined in claim 2 and wherein the proportions of sulfur trioxide to said combined steam and oxygen mixture is based on the rate of acid generated by the oxygen with the sulfide minerals less the acid consumed by acid reaction with gangue minerals.

4. The process as defined in claim 1 and wherein steam and oxygen, optionally with sulfur trioxide, are injected in one wellbore; and the lixiviant is withdrawn from an adjacent wellbore.

5. The process as defined in claim 1 and wherein said sulfidic metal ore is a prophyry copper sulfide ore body.

6. The process as defined in claim 1 and wherein said sulfidic metal ore is chalcopyrite.

7. The process as defined in claim 1 and wherein said sulfidic metal ore is a mixture of pyrite and chalcopyrite.

8. The process as defined in claim 1 and wherein the primary copper bearing sulfide is chalcopyrite with minor amounts of other copper sulfides.

9. The process as defined in claim 1 and wherein the injection and lixiviant recovery is from the same well.

10. The process as defined in claim 1 and wherein additional amounts of sulfur dioxide are injected in said well bore. 

1. IN A PROCESS FOR RECOVERING METAL VALUES BY IN-STU MINING OF SAME, THE STEPS COMPRISING: DRILLING AT LEAST ONE WELLBORE IN AN ORE BODY HAVING METALLIC SULFIDE VALUES, INJECTING MIXTURE OF STEAM AND OXYGEN IN SAID WELLBORE IN PROPORTIONS OF AT LEAST 25:1 RATIO BY WEIGHT OF STEAM TO OXYGEN AS A GAS WHEREBY THE GAS IS AT LEAST AT THE HYDROSTATIC BUT BELOW THE ROCK PARTING PRESSURE REQUIRED FOR A GAS INJECTION AND THE TEMPERATURE, DEPENDING ON THE COMPOSITION, IS CONSISTENT WITH THE INJECTION PRESSURE, SAID IN-SITU MINING BEING CARRIED OUT AT A DEPTH PREFERABLY BENEATH THE WATER TABLE, MAINTAINING A MIXTURE OF STEAM AND OXYGEN IN AN INTIMATE CONTACT IN SAID ORE BODY ADJACENT SAID WELLBORE, OXIDIZING SAID METAL SULFIDES IN SAID ORE BODY WHEREBY SULFUR, SULFUR DIOXIDE AND/OR TRIOXIDE ARE FORMED AND SUBSEQUENTLY CONVERTED TO SULFURIC ACID WITH SAID STEAM, SAID SULFUR DIOXIDE AND/OR TRIOXIDE THEREBY FORMING AN ACID LIXIVIANT SOLUTION IN SAID WELLBORE, MAINTAINING SAID LIXIVIANT SOLUTION IN INTIMATE CONTACT WITH SAID METAL VALUES IN SAID ORE FORMATION, AND RECOVERING SAID METAL VALUES TOGETHER WITH SAID LIXIVIANT AND CONDENSED STEAM.
 2. The process according to claim 1 and wherein additional amounts of sulfur trioxide are injected in said wellbore in admixture with steam and oxygen.
 3. The process as defined in claim 2 and wherein the proportions of sulfur trioxide to said combined steam and oxygen mixture is based on the rate of acid generated by the oxygen with the sulfide minerals less the acid consumed by acid reaction with gangue minerals.
 4. The process as defined in claim 1 and wherein steam and oxygen, optionally with sulfur trioxide, are injected in one wellbore; and the lixiviant is withdrawn from an adjacent wellbore.
 5. The process as defined in claim 1 and wherein said sulfidic metal ore is a prophyry copper sulfide ore body.
 6. The process as defined in claim 1 and wherein said sulfidic metal ore is chalcopyrite.
 7. The process as defined in claim 1 and wherein said sulfidic metal ore is a mixture of pyrite and chalcopyrite.
 8. The process as defined in claim 1 and wherein the primary copper bearing sulfide is chalcopyrite with minor amounts of other copper sulfides.
 9. The process as defined in claim 1 and wherein the injection and lixiviant recovery is from the same well.
 10. The process as defined in claim 1 and wherein additional amounts of sulfur dioxide are injected in said well bore. 